For reasons of comfort and safety, modern vehicles use electrically heatable vehicle seats. The drivers seat, as well as the other seats in the vehicle, can be so arranged that they can be heated by means of special heating elements which comprise electrically conducting wires, which are placed in the shape of heating coils in each seat. Such a heating element is normally placed in the cushion and the back-rest of the seats when they are manufactured. Furthermore, the heating element is connected to a current feeding unit which delivers current. In this way, the heating element can be heated to a suitable temperature.
One method of manufacturing vehicle seats with heating elements is to first place the heating element in a tool intended for manufacturing the padding of the seat. This manufacturing is normally done by means of so-called cold foaming, which is a previously known manufacturing method for producing soft elastic plastic foam. When using this method, the heating element is first placed in the tool, subsequent to which the components of the cold foaming process are supplied and are allowed to react in such a way that the padding of the seat can be shaped.
A problem in connection with cold foaming is that there is a risk that the heating element is moved from its intended position in the cold foaming tool. To be more exact, there is a risk that the heating element is moved both when the components which are part of the process are supplied to the tool and also during the cold foaming process itself, when the foam "ferments" and is shaped by the tool.
In order to solve this problem, the heating element can be equipped with surrounding supporting materials. From U.S. Pat. No. 4,869,550, a heating element is previously known which comprises a supporting material and an isolated heat conductor which is secured to the supporting material by means of securing organs in a pattern, and which covers a surface of the supporting material so that the supporting material forms a net-pattern with large meshes of essentially electrically isolating perforated material. During manufacture, the heating element with its supporting material is placed in a negative form for a vehicle seat. The form is filled with plastic foam in such a way that the heating element with its supporting material is surrounded by said plastic foam. Points of binding material on the supporting material of the heating element attach the heating element to the upholstery material of the vehicle seat and prevent it from being moved out of place. The shaping of the flat heating element with its net-shaped supporting material enables the plastic foam to penetrate and surround the heating element with an essentially constant density of the stiffened plastic foam. In this way, hardened portions in the foam, and uneven heating of the vehicle seat can be avoided.
In connection with conventional manufacturing of heating elements with net-shaped supporting materials, the supporting material is delivered in the form of rolls or sheets. Pieces of material with the desired dimensions are die-cut from the supporting material. This die-cutting of pieces of supporting material for individual heating elements generates large amounts of waste in the form of pieces of supporting material which are not used, thus causing unnecessary costs. With contemporary technology, 20-25% of the supporting material is lost as waste, which of course is a drawback when using this previously known manufacturing method.
Another drawback when using heating elements with net-shaped supporting materials is that the user perceives a certain stiffness or inflexibility of the manufactured vehicle seat. When manufacturing the vehicle seats, such a net-shaped supporting material can also be too stiff to allow the heating elements to be shaped to the shape of the seat in a desired manner and to be fixed in the desired position. For this reason, it might be desirable to die-cut chosen portions which are located inside the outer contour of the pieces of supporting material as heating elements, in order to obtain portions without supporting material, which facilitates shaping and folding of the heating element. Such a die-cutting will also result in unwanted pieces of unused supporting material.
A further drawback of net-shaped materials is that they have a tendency to interfere with the foaming process and to cause, for example, blisters in the resulting padding material.
The above-mentioned drawbacks are normally caused by the net-shaped supporting material being too densely formed. However, the supporting material must have a certain density, and can thus not be shaped with an arbitrary sparsity if the desired properties are to be obtained.